call on/upon sb (to do sth)
(a) formally invite or request sb (to speak, etc) 郑重邀请或要求某人(讲话等)
I now call upon the chairman to address the meeting. 现在请主席致辞.
(b) appeal to or urge sb (to do sth) 恳求或促使某人(做某事)
We are calling upon you to help us. 我们恳求你帮助我们.
I feel called upon (ie feel that I ought) to warn you that.... 我觉得我应该提醒你....